The writer seems to be making fun of the way Lieutenant Canby dresses. I'm not sure whether the idea is "cowboy" or "cavalry," but it refers to someone who rides a horse. I will say "cowboy." He is saying that Canby is wearing cowboy boots with spurs. A spur is a little round wheel with points that is fastened to the heel of a cowboy boot. The cowboy kicks his horse with his heels to make the horse go faster. The spurs make the horse feel the kick. This is called "spurring the horse on." Here is a picture: https://imgur.com/t0e364s Canby does not ride a horse. He pilots an airplane. He just wears spurs because he likes to look like a cowboy. The writer is asking "Why does he need spurs? That's silly." Instead of saying directly "that's silly," the writer is being sarcastic. "Presumably" means "we don't know, but it is the only possible guess." The writer is saying "he needs those spurs to make his aeroplane go faster." We understand that this is a joke. You can't spur an airplane. He doesn't need the spurs, he is pretending he is something that he isn't. In the US, a cultural idea is that cowboys are strong and manly, so he wants to dress like a cowboy even though he isn't one. Even the phrase "spurs... gleaming" is probably a joke. It suggests that the spurs still look new and have never been used. (Maybe real cowboys keep their spurs polished and gleaming, but I doubt it). By the way, the spelling "aeroplane" is out of date. The current spelling is "airplane." This reflects pronunciation. In the days when it was spelled "aeroplane" it was pronounced as three syllables, now it is pronounced with only two.

From F. Scott Fitzgerald's "The Last of the Belles" in the Saturday Evening Post - March 2, 1929. Spurs are used to make a horse go faster. (I don't use them on horses myself.) This is an analogy used to compare the above situation with a horse rider using something to cause forward motion. Something (I can't tell what, from this passage) was used to move his airplane forward. I am really not sure! I wonder if it is referring to the bars on a uniform, indicating his piloting skills? Well, after I posted, I saw the other two responses. They obviously understood it as literal, rather than analogous. The writer included a joke, and I didn't catch it. LOL

He was wearing spurs, as used by horsemen to make their horses go fast. The writer clearly finds this ridiculous, imagining that the lieutenant was stupid enough to try kicking his aeroplane with them.